I 



STERILIZATION 191 



n 



so that the extraordinary result was realized of obtaining at the 

 sewage works an effluent which had a greater bacterial purity 

 than the town water-supply. 



The general conclusion was that, with a good effluent, sterility 

 can be insured by the addition of about 5 parts per 100,000 of 

 available chlorine. If removal of coli and enteritidis only is 

 aimed at, one-tenth of this amount ( = 0*5 part), or sometimes 

 even less, is sufficient, as would be the case in a discharge near 

 shell-fish gathering-grounds, into watercress beds, or into rivers 

 that are used as a source of supply by water companies. 



Absolute Sterility. — Even the tertiary effluent contained some 

 thousands of spores per c.c, of which forty to fifty are capable 

 of resisting the temperature of boiling water for several 

 minutes ; therefore I soon found that absolute sterility was 

 not practical, not merely on account of the cost of the large 

 quantity of disinfectant required, but also because the residual 

 disinfectant would be inadmissible in an effluent. For this 

 reason it became necessary to discover what the highly- 

 resistant organisms were, and particularly whether they could 

 be injurious. I found that they were constant in character 

 throughout the sewages and effluents, and consisted of a group 

 of bacteria of the hay-bacillus type, non-pathogenic, not pro- 

 ducing smell, and of great assistance towards the resolution of 

 organic matter. Absolute sterility, therefore, is not required, 

 and, if attained, would not be maintained. 



When chlorine or its oxy-compounds are used as sterilizers, 

 the cost of their production becomes important. It is obvious 

 that the economy of a process will be determined by the 

 quantity of " available chlorine " produced in a continuous 

 process for a given expenditure of electrical energy, or, in 

 other words, the cost of electrolytic chlorine per kilo, in com- 

 parison with chloride of lime, hypochlorous acid, and free 

 chlorine obtained chemically.^ 



In the Digby process, which I have recently had occasion to 

 ritically examine, the costs are reduced to a minimum by 

 insuring a greater percentage of free chlorine being produced 

 from a given quantity of salt. This is attained by enclosing 

 the electrodes in asbestos diaphragms, which prevent the salt 

 solution from passing with the liberated ions from the cell. In 



^ As to cost of chlorine electrolytic plant, see Haussermann, Dingier' s Polyt. 

 Journ., 1895, 296, p. 189 ; Schoop, Zeits. f. Electrochcmie, 1895, ii. [10], 209 ; 

 Electrical Review, 1898, April 29. 



